Liquid crystal displays (LCDs) generally display images by transmitting or blocking light through the action of liquid crystals. An LCD includes an array of pixels for displaying images. LCDs have been used in a variety of computing displays and devices, including notebook computers, desktop computers, tablet computing devices, mobile phones (including smart phones) automobile in-cabin displays, on appliances, as televisions, and so on. LCDs often use an active matrix to drive liquid crystals in a pixel region. In some LCDs, a thin-film transistor (TFT) is used as a switching element in the active matrix.
Certain LCDs operate in a fringe field switching (FFS) mode. FFS mode LCDs may have better aperture ratios and transmittances than in-plane switching (IPS) mode LCD devices. IPS LCDs generally use thin film transistor (TFT) technology to improve image quality. By contrast, in a FFS LCD, a common electrode and a pixel electrode are formed of transparent conductors, and the distance between the common electrode and the pixel electrode is maintained at a relatively narrow range to drive liquid crystal molecules by using a fringe field formed between the common electrode and the pixel electrode. FFS LCDs may deliver brighter picture and have better color consistency than IPS LCDs, and may deliver these qualities at relatively wide viewing angles.
Typically, display pixels are addressed in rows and columns, which may reduce the connection count from millions for each individual pixel to thousands, when compared to a display having pixels addressed only by rows and/or columns. The column and row wires attach to transistor switches; one transistor is present for each pixel. The one-way current passing characteristic of the transistor prevents the charge applied to the pixel from draining between refreshes of the display image.
Stability of the common electrode voltage (Vcom) may become more important as the resolution of the LCD increases, since the Vcom voltage level directly affects the luminescence and luminescence uniformity of the LCD. For example, pixel coupling may cause a ripple in Vcom voltage, which in turn may cause a perceptible color shift in the display. For example, the display may have a greenish tint or hue.
Effective methods for stabilizing Vcom include decreasing parasitic coupling capacitances between a common electrode and a pixel electrode and reducing a resistance of the common electrode. The common electrode is normally formed of a transparent conductive material, such as indium-tin oxide (ITO). One way of reducing the resistance of the common electrode is to increase the ITO film thickness. Another way of reducing the resistance of the common electrode is to add a metal layer to the ITO film. The metal layer usually forms a gate electrode. Alternatively, the metal layer may also be formed by a different metal layer referred to as a “third metal layer,” to decrease Vcom resistance and increase aperture ratio, where a gate electrode of the TFT is formed of a first metal layer and the source/drain electrodes of the TFT are formed of a second metal layer. However, the addition of the third metal layer may produce rubbing mura, which may impact performance of an LCD. Generally, “rubbing mura” is an unevenness or irregularity in alignment of liquid crystal molecules, which may cause uneven changes in luminance across the surface of the display.
Therefore, there remains a need for developing techniques for improving stability of the common electrode and producing a rubbing mura-free third metal layer in FFS TFT for LCDs.